1. Field of the Invention
This invention relates to a tunable dispersion compensation apparatus that suppresses the effects of the transmission channel dispersion which impedes transmission when optical signals used in high-speed communications are transmitted across long distances among various points.
2. Description of the Prior Art
In data communications systems for the Internet and mobile telephones, packet-switched systems are used instead of the circuit-switched systems used in conventional fixed voice telephony. In packet-switched systems, it is necessary to adjust the timing of the large numbers of packets arriving at a switch so that they do not collide with each other, and transmit the packet signals to their respective destinations. In the case of high-speed systems that use optical fiber for transmission in particular, in existing packet-switched systems, the optical signals are first converted to electrical signals and stored in semiconductor memory devices. The appropriate optical fiber is selected depending on the destination, and the signals are converted from electrical signals back to optical signals and transmitted at the desired timing. However with this method, the operating speed of the portions that process electrical signals is limited, so there is a problem in that the high capacity, high-speed advantages of optical signals cannot be fully exploited.
In contrast, attempts have been made to achieve control over the timing and destination of optical signals as is. Random access memory that is able to store optical signals at high speed has yet to be achieved as of now, so the method used is to control timing by switching among delay lines that use large numbers of optical fibers of different lengths (taking advantage of the time lag when light passes through the fiber), thus constituting a buffer memory. With this method, control can be achieved without converting optical signals into electrical signals, so ultra-high-speed optical transmission of several gigabits per second or more can be handled. However, there are drawbacks in that optical amplifiers are required to compensate for the losses in delay lines that use large numbers of optical fibers, and the structure becomes complicated.
In regard to this, JP-A 2001-209082 discloses a tunable optical delay apparatus that has a wavelength conversion apparatus and optical amplifier for compensating for loss within a single optical fiber loop, and is provided with a band-limited type optical input/output port for taking light out of and into the loop. The band-limited type optical input/output port consists of a band-limited optical filter sandwiched between two circulators. The band-limited optical filter is made of a fiber Bragg grating (FBG). The input light enters the fiber loop through an optical port and circulates. The optical signal, with its wavelength converted by an optical single-sideband (SSB) modulator, circulates around the loop while its wavelength is shifted each lap until it is extracted when the wavelength falls outside the reflection band of the optical input/output post.
In addition, with the tunable optical delay apparatus disclosed in JP-A 2001-209082, the characteristics of the FBG of the input/output port have a major effect on the overall tunable optical delay. Light within the reflection band is reflected by the FBG and returned to the loop, but the FBG has dispersion characteristics, so the delay time when light is reflected may vary greatly depending on the wavelength of light.
When long-distance, high-speed transmission is performed, waveform deterioration typically occurs due to the effect of dispersion within the optical fiber. The effect of dispersion within the fiber fluctuates with disturbance, so the development of compensating technology able to deal with it is proceeding. Moreover, each optical signal in a packet-switched system originate in a various source point, so the propagation distance of each is not necessarily the same and thus it is necessary to vary the amount of compensation for dispersion for each optical signal.
An object of this invention is to provide a tunable dispersion compensation apparatus that has a wide range of variability and is capable of a large amount of compensation.
Another object of this invention is to provide a tunable dispersion compensation apparatus that suppresses the effects of the wavelength dispersion characteristics of a transmission channel by compensating for the wavelength dispersion of the transmission channel prior to the transmission of an optical signal, or by compensating after the optical signal is received.